Avoiding the Clever Hans Phenomenon in Scent Detection

Clever Hans was presented to an audience in 1904 Edited photo of Clever Hans being presented to an audience in 1904 (Source image: Clever Hans, by Karl Krall (Karl Krall, Denkende Tiere, Leipzig 1912, Tafel 2) [Public domain], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3ACleverHans.jpg)

This blog has been updated in the IAABC Journal, Spring 2020: https://spring2020.iaabcjournal.org/the-clever-hans-effect-scent-detection/?fbclid=IwAR28KpQBKKbCRY5GyAy73ssB2pWiEyyokkwxYJIZ0jHDeN556UsObQ3fHhg

Clever Hans was a horse who became famous for answering math questions by counting with his hoof (1). For example, when asked what’s the square root of nine, he tapped his hoof three times. He was featured by the New York Times and purported to be 90% accurate. A sceptical investigator eventually realized that Clever Hans was simply a master at reading the body language of the people posing the questions. The “precondition for Hans’ correct answer was that the questioner had to know the answer and Hans had to be able to see him”(2). He tapped his hoof until he received body language cues to stop. Without assistance, Hans couldn’t perform the feats that made him a star. Even knowing this, one researcher reported being unable to control his body language sufficiently to prevent the horse from reading him.

The Clever Hans phenomenon is important to proof against when training sniffer dogs. Even when you know about this issue, it’s difficult to avoid when a person who knows the hide location is in the room! If you unintentionally assist your sniffer dog in training, you will have trouble handling blind searches, because when you don’t know where the scent is hidden you can no longer help your dog. Unless your dog learns to find source independently using scent, he may become reliant on outside assistance and fail when that assistance disappears.

So what would this look like? Andy & Solo to the rescue with a demonstration!

In the first segment, the handler shows his dog where the hide is by:

  • 0:04 Quickly passing by the first couple of objects and blocking the dog from moving backwards by standing still, tight to the wood
  • 0:06 Stopping near the hidden scent
  • Facing his shoulders, hips, and toes towards source and
  • 0:11 Gesturing towards the hide with his hand and waiting for his dog to find it.

In the next segment 00:20, the search is shown with far less assistance, while the handler is seated at a distance.

Have you considered how your body language may be leading your sniffer dog to source? To avoid the Clever Hans phenomenon when training your sniffer dog, try:

  • Not staring at the hide
  • Following behind your dog’s butt, keeping your eyes focused on wherever he’s searching (whether it’s hot or cold)
  • Walking at the same pace until your dog finishes finding and indicating source (see https://youtu.be/E4NUAnPO_kk?t=31)
  • Practicing blind searches sitting on a chair at a distance, with your hands crossed behind your back while your dog searches off leash https://wp.me/p74GTg-hO
  • Staying in another room, handling blindfolded (see https://youtu.be/6aud5J3S8Ec) , or turning the lights off (as long as you modify the environment for safety), or
  • Practicing double-blind searches – Double-blind tests are the gold standard used in canine scent detection testing and research to reduce or eliminate bias. In double-blind searches (see https://youtu.be/HMjH36h2PwE), an experienced person places hides and leaves until the test is completed, to prevent (intentionally or unintentionally) communicating its position to the team during the search. (Usually the number and placement of hides is determined by rolling dice.) A proctor is present during the test to ensure teams don’t break the rules (such as having the handler find hides by visual inspection). The proctor has no knowledge about the hides and has no vested interest in the outcome. The canine handler does not know how many hides are present or where they are. The handler receives no feedback about whether they are right or wrong until the results are written down. Once all of the results are compiled, the person who placed the hides determines whether teams passed or failed.

Try a Clever Hans-inspired challenge and let us know how it goes in the comments below.


Learn more in our online Scent Detection Foundation course at https://store.huntersheart.com/Scent_Detection_Foundation_Online_Dog_Training_C_p/sdf.htm.

  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3921203/
  2. Egnor Michael (2015). The Clever Hans Problem and Research on Animal Cognition. Retrieved from https://evolutionnews.org/2015/12/the_clever_hans/.
  3. Source image: Clever Hans, by Karl Krall (Karl Krall, Denkende Tiere, Leipzig 1912, Tafel 2) [Public domain], via Wikimedia Commons. Retrieved from https://commons.wikimedia.org/wiki/File%3ACleverHans.jpg.
About Dr. Carla Simon (132 Articles)
From bed bugs to birds, from narcotics to nosework, Dr. Carla Simon, MD, BSc, MBA has trained thousands of students around the world, since 1999.

Questions and comments are welcome

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: